154 exposure to low level microwave radiation has been linked with headaches,
sleeplessness, irritability, fatigue and memory loss. Microwaves are widely used in applications like wireless computing and
mobile phone networks. Much public concern has been raised about the possibility of serious long-term health effects such as cancer. As yet,
research has failed to demonstrate such a link conclusively.
16.2.5 Other Effects of Non-Ionising Radiation
Ozone may be produced as a result of electrical discharges or ionisation of the air surrounding non-ionising radiation sources, e.g. UV, high power laser,
microwave, and short duration exposure in excess of a few tenths ppm can result in discomfort headache, dryness of mucous membranes and throat.
16.3 Evaluation of Non-Ionising Radiation
Portable hand-held meters are available to measure NIR. They incorporate a suitable photo emissive material e.g. UV, visible or IR so that incident
radiation releases electrons from the surface. These electrons are collected by an anode and made to flow as an electric current which is measured by a
suitably calibrated ammeter see below. The radiation data obtained are assessed against appropriate occupational
exposure limits. In fact ACGIH has adopted or proposed TLVs for each of the following:
Ultraviolet radiation Visible and near infrared radiation.
Laser radiation. Microwave and radio-frequency radiation.
The radiation intensity limits are expressed in mWcm
2
.
16.4 Lighting 16.4.1 Recognition
The visible radiation portion of the electromagnetic spectrum is narrow, ranging between 400 and 700 nm. It is the sensitivity of the eyes to this visible
155 radiation that enables us to see. In terms of occupational hygiene we are
concerned with the subjective feeling of visual comfort, and good illumination which is described in terms of the quantity and quality of the lighting.
Quantity - this is the amount of illumination on the task. It is measured in lux
and must be sufficient for the worker to undertake the task.
Quality - is the suitability of the illumination, for example the distribution of
brightness in a visual environment, the colour of light, its direction, diffusion and the degree of glare
The least desirable type of lighting is that from a single bulb in the middle of the room. Decreased contrast and improved visibility will result from
increasing the number of lighting sources across a ceiling. In general, for each visual task performed, a certain minimum quantity of light
arriving on each unit area of the object in view is required, dependent primarily upon the nature of the work that is being undertaken. Too little light can lead
to eyestrain and headaches, too much light can result in glare. Guidance on the recommended service values of illumination are given in the Code of the
Institution of Building Services Engineers CIBSE Code in the UK, and by the American Society of Heating and Ventilation Engineers ASHRAE in the US.
Lighting in the various areas of factories and offices can be classified according to three categories:
Local lighting Localised lighting
General lighting.
Research has shown that favourable lighting conditions exist when the illumination of the task is about three times greater that of its immediate
surroundings, and when the immediate surroundings have about three times the illumination of the general workroom. Good lighting has a beneficial
psychological effect on a workforce and its productivity.
156
16.4.2 Evaluation of Illumination
The instrument most commonly used for the measurement of illumination is a photoelectric light meter often termed a Lux meter. When light is incident
upon the photoelectric cell, the energy in the radiation is converted into electrical energy and the current produced recorded on a meter calibrated in
lux. It has a built-in filter which automatically applies the necessary correction factor when daylight, mercury lamp light or fluorescent light is to be measured,
and is colour corrected to respond to the human eye. The quantitative results obtained are assessed in terms of appropriate guidance criteria such as those
recommended by CIBSE or ASHRAE.
16.4.3 Glare
Glare may be defined as any brightness within the field of vision where such character would cause discomfort, annoyance, interference with vision, or eye
fatigue. Three different types of glare may be present separately or in combination.
Disability glare this will affect the capacity to see clearly, e.g. the undipped
headlamp on a car or sunshine reflecting from a wet surface.
Discomfort glare this effect increases with time, e.g. a part of a visual scene
windows by day, lighting by night may be too bright compared to the background.
Reflected glare this is seen in shining or polished surfaces which reflect a
more or less distorted image of a bright light, fitting or window. This can be annoying or disabling, for it may be difficult or impossible to see whatever is
beneath.
16.4.4 Good Illumination
General guidelines for designing illumination of sufficient quantity and suitable quality are:
Consider lighting at the design stage of any building or workplace
157 Design for sufficient lighting levels in line with established guidance
such as the CIBSE code‟ Integrate daylight and artificial light
Avoid glare Minimise flicker
Ensure adequate maintenance of glazing surfaces and light fittings.
158
17 IONISING RADIATION
17.1 Nature
It is possible to explain many atomic scale phenomena by assuming that all atoms are made up of three fundamental particles. These are called electrons,
protons and neutrons. The simplest atomic combination is formed by one electron and one proton - the hydrogen atom. In general, however, a number
of negatively charged electrons rotate in certain allowed orbits around a central nucleus which is composed of an equal number of positively charged
protons and some neutrons. The neutrons carry no charge and the equal number of electrons and protons ensure the charge neutrality of the complete
atom, as their charge is equal in magnitude but opposite in sign. The diagram below illustrates this for three variants of the hydrogen atom,
which have different numbers of neutrons. Such variants are called isotopes.
Hydrogen Hydrogen
Source: Modified from Dirk Hünniger, licensed under Creative Commons Attribution ShareAlike 3.0
Figure 17.1 - Hydrogen Isotopes
Ionising radiation refers to particles or electromagnetic radiation which have sufficient energy to affect atoms directly i.e. ionise them, namely to create
charged particles, or ions, when they interact with matter. There are five different types of ionising radiation, namely alpha
, beta , neutrons n,
159 gamma , X-ray
χ. The first three of these are particles and the latter are examples of electromagnetic radiation. Details are given in the Table set out
below.
Type Symbol
Nature Charge
Relative Mass
Range in Air
Penetration
alpha
particulate helium nucleus
+ + 4
0.4 - 2 cm None
beta
particulate electron
- 11800
5-20 cm Slight
neutron n
particulate neutron
1 long
High gamma
electro-magnetic
v.long High
x-ray
electro-magnetic v.long
High
17.2 Radionuclides
